1. Field of the Invention
The present invention relates to an image forming apparatus, a method of controlling this apparatus and an image forming system.
2. Description of the Related Art
An image forming apparatus to which a sheet feeding unit can be added on as an optional unit in order to increase the capacity for stacking a sheet material has been proposed. Such an optional unit executes the sheet feeding operation in response to a feeding instruction transmitted from the engine controller of the image forming apparatus. The engine controller terminates the sheet feeding operation when a sensor provided on the optional unit senses the leading edge of a sheet. If the leading edge of a sheet cannot be sensed by the sensor despite the fact that sheet feeding has been instructed, on the other hand, then the engine controller causes the optional unit to retry the sheet feeding operation (see the specification of Japanese Patent Application Laid-Open No. 2005-272083).
Assume here that the optional unit transmits status data to the image forming apparatus by serial communication. Assume also that the result of sensing by the sensor provided on the optional unit to sense a sheet also is transmitted as status data.
When optional units are provided in multiple stages, however, the status data sent by serial communication is delayed and there is the danger that this will hamper an increase in the sheet conveying speed. That is, if the optional unit is provided in multiple stages and with higher functionality, there is an increase in optional-unit status data to be checked by the engine controller. As a result, there is a widening of the update interval of the status data that includes the result of sensing by the sensor and, hence, the real-time nature of the status data is lost.
On the other hand, the higher the sheet conveying speed is made, the shorter the time between sheets becomes during continuous printing and, hence, the more difficult it becomes for the sensor to sense the sheet-to-sheet interval. Here the “time between sheets” refers to the difference between the times at which at the trailing edge of a preceding sheet and the leading edge of the succeeding sheet pass by a prescribed position. Further, the “sheet-to-sheet” interval refers to the interval between the trailing edge of a preceding sheet and the leading edge of the succeeding sheet.
In order to solve this problem, it will suffice to enlarge the sheet-to-sheet interval in such a manner that the sensor can sense the sheet-to-sheet interval reliably. However, this will lower the maximum throughput of the image forming apparatus. On the other hand, if a dedicated signal line separate from a serial signal line is provided and the sensor information is sent to the image forming apparatus via this line, throughput can be maintained. However, this can lead to higher cost.
Accordingly, the present invention seeks to solve one of these problems or other problems. For example, the present invention seeks to provide an image forming apparatus in which throughput can be maintained without increasing the number of signal lines. Other problems will be understood from the entirety of the specification.